Doorbells are widely used in buildings, such as to enable visitors to announce their presence. FIG. 1 schematically illustrates a prior-art doorbell system 100 including a stepdown transformer 102, a doorbell switch 104, and a mechanical doorbell sound output device 106. A primary winding 108 of the transformer 102 is electrically coupled to an alternating current (AC) electrical power source (not shown), and a secondary winding 110 of the transformer 102 is electrically coupled in series with the doorbell switch 104 and the doorbell sound output device 106.
The doorbell switch 104 is normally open, and electrical current therefore normally does not flow through the mechanical doorbell sound output device 106. When the doorbell switch 104 is pressed such that the switch is closed, electrical current flows through and energizes the mechanical doorbell sound output device 106. Typically, this action produces a “ding” sound. When the doorbell switch 104 is released such that the switch is opened, a plunger of the mechanical doorbell sound output device 106 strikes a lower pitched metal plate, producing a “dong” sound. The mechanical doorbell sound output device 106 is schematically depicted herein as a solenoid coil, and mechanical aspects are not shown to promote illustrative clarity.
FIG. 2 schematically illustrates another prior-art doorbell system 200. The doorbell system 200 is similar to the doorbell system 100 of FIG. 1, except that the mechanical doorbell sound output device 106 is replaced with an electronic doorbell sound output device 206, and a semiconductor diode 212 is electrically coupled in parallel with the doorbell switch 104. The semiconductor diode 212 provides half-wave rectified direct current (DC) electrical power to the electronic doorbell sound output device 206, thereby enabling the doorbell sound output device 206 to be ready to receive a trigger signal. When the doorbell switch 104 is closed, the electronic doorbell sound output device 206 is triggered by the presence of an AC electrical current waveform flowing through the doorbell sound output device. When the doorbell switch 104 is released, DC electrical power is again provided to the electronic doorbell sound output device 206, thereby electrically powering the electronic doorbell sound output device 206 and enabling the doorbell sound output device to continue to play a sound output device melody lasting a few to several seconds, even if the trigger signal is shorter than one second. Many electronic doorbell sound output devices on the market today operate in this manner, requiring a diode (e.g., the diode 212) connected across the doorbell switch.